In the main portion of their range, Mountain Beavers occur in moist forests and forest openings, where they prefer cool, moist environments such as overgrown thickets and seepage areas. They are most abundant near water courses in early to mid-seral stages vegetated by a tangle of secondary growth tree species, shrubs and forbs, and containing debris left from earlier forests (Carraway and Verts 1993). It prefers damp soils, digging networks of tunnels along stream banks. Tunnels generally are just below the ground surface, usually on north slopes in California, and on south slopes in British Columbia. It is primarily fossorial but can climb trees and swims well (but not arboreal or aquatic), and stays mostly underground in winter. Oval nests are constructed with leaves, twigs and grasses in a chamber that may be about two feet below the surface of the ground.

Aplodontia rufa nigra occurs only in sheltered gulches, steep north facing slopes, and in a few old dune systems, where it requires soil conditions that allow easy excavation and abundant succulent plant food. Aplodontia rufa phaea requires north-facing slopes with dense thickets of brush typically composed of coyote brush, sword fern, blackberry, and poison oak. Openings within Bishop pine forest with dense herbaceous understorey of sword fern and blackberries are also occupied (Carraway and Verts 1999).

Mountain Beavers feed on a wide variety of vegetation. They consume ferns, forbs, and deciduous plants in summer and conifer foliage in fall/winter if other plants are unavailable (Banfield 1974). It forages mainly above the ground (Epple et al. 1993). They require free surface water or succulent vegetation on a daily basis and caches grasses and forbs for winter food. They are active during winter. Throughout the 24-hour day in summer, five to seven periods of activity alternate with periods of rest. They are more active at night than during daylight hours.

Mountain Beavers have a low rate of reproduction and are monoestrous. Gestation lasts 28-30 days. One litter of two to four altricial young are born March-April, sometimes as late as early May in the north. Young are weaned in about six to eight weeks. Females are sexually mature in about two years; yearling females may ovulate but do not breed (Carraway and Verts 1993). A few live up to five to six years. They are usually solitary but may live in loose colonies. Population density estimates generally range from four to eight per hectare, but up to 15-20 per ha (Carraway and Verts 1993). The home range of ten adults radio tracked for three to 19 months was 0.03-0.20 ha, with a mean of 0.12 ha (Carraway and Verts 1993). Significant predators of Mountain Beavers include Coyote and Bobcat.

The Mountain Beaver occurs along the Pacific coast of western North America, from southwestern British Columbia, Canada, south to central California in the United States. It ranges from near Merritt, British Columbia, south along the Cascade, Olympic, Coast, and Siskiyou ranges to Rio Dell, California; Mt Shasta, California, southeastward through the Sierra Nevada of eastern California and west-central Nevada; Point Arena, Mendocino County, California; and near Pt. Reyes, Marin County, California (Carraway and Verts 1993).

The subspecies nigra has the most restricted distribution of all subspecies, it is known only from an area of 60 km² in the vicinity of Point Arena, Mendocino County, California. The subspecies phaea is known primarily from an area of approximately 285 km² within the Point Reyes National Seashore, Marin County, California, with a few populations occurring on privately owned land immediately adjacent to the Park.

In California, relict populations are protected in National Park Service land or through state endangered species listing. The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) have the mountain beaver listed as; Special Concern (01 Nov 2001). The subspecies nigra of northern California is protected nationally under the United States Endangered Species Act as of 1991.